1. Field of the Invention
The present invention relates to an image forming apparatus having an image carrier for forming an electrostatic latent image thereon, and a plurality of developing units facing the image carrier and individually movable toward and away from the image carrier. More particularly, the present invention is concerned with an image forming apparatus which allows each developing unit to be shifted between a developing and a non-developing position surely and easily.
2. Discussion of the Background
An image forming apparatus of the type having a plurality of developing units located to face a photoconductive element or similar image carrier is extensively used today. A color electrophotographic copier, printer or facsimile machine, for example, develops a latent image by moving a desired developing unit toward a developing position defined on an image carrier while moving the other developing units or only developing rollers thereof away from the developing position. With this implementation, it is possible to prevent a toner image produced by one developing unit from being disturbed by the other developing units. For example, assume there is a toner image developed by a developing unit which is located upstream of the others with respect to an intended direction of rotation of the image carrier. Then, without such an implementation, the toner image would be disturbed by toners being deposited on the developing rollers of the other developing units. The above implementation also serves to prevent toners of different colors stored in the individual developing units from being mixed together.
Various approaches for moving a developing unit itself toward and away from an image carrier have been proposed, as disclosed in Japanese Patent Publication No. 55-3707 and Japanese Laid-Open Patent Publication No. 52-18332, for example. An approach for moving a developing roller alone is taught in Japanese Laid-Open Patent Publication No. 52-18332. However, the prior art schemes which move the whole developing unit need springs or similar biasing members for yieldably maintaining the individual developing units in a predetermined position, and an exclusive driver for moving the individual developing units toward and away from the developing position, resulting in a complicated construction. The prior art schemes of the kind for moving a developing roller only as stated above have a drawback in that it is difficult to transmit a driving force to the developing roller because the developing roller itself is movable within the developing unit.